Aluminum oxide is a natural nanofilm

Metallic aluminum is extremely reactive with the oxygen available in air. If exposed to the air, metallic Al forms a thin film of oxide, called alumina. As the oxide forms it protects the metal from further oxidation since the alumina film is practically impenetrable for oxygen.

According to Timothy Campbell and coauthors [PRL v.82, p.4866 (1999)], within first 100 picoseconds of exposure to air about 4 nm of oxide forms on the metallic Al surface. Numerical simulations show that the oxide thickness increases linearly with time during the first 50 ps and subsequently the rate becomes smaller and the thickness saturates at 3.3 nm. Previous experiments showed that aluminum nanoclusters form oxide films from 3 to 4 nm thick in oxygen gases at room temperature. Thus there is a high degree of similarity between Campbell's simulations and experiments. The main conclusion from this analysis is that high quality nanometer-scale films of aluminum oxide can be obtained naturally.

Such films have been used in nanotechnology. For example, such extremely thin films of alumina were instrumental for understanding the phenomenon of superconductivity. Giaever made electrical junctions with the aluminum oxide, now called Giaever junctions, and discovered that superconductors, unlike normal metals, have a small energy gap in the spectrum of their electrons. In fact, this energy gap gives superconductors their mysterious ability to carry electrical current without any dissipation or losses. Giaever won the Nobel Prize in 1973 for his discovery.

Aluminium oxide was taken off the United States Environmental Protection Agency list of chemicals in 1988. The U.S. But it is still listed in the EPA's TRI list, but only in its fibrous form.

The picture is the magnified image of the structure of an ordinary aluminum foil.